As a result of continuous advance of technology, evolution of various electronic devices also becomes more and more quickly. Generally, headphones are used as voice transmitting tools between electronic devices and user's ears. Except wearing conformability, voice quality is also a more important consideration during the development of headphones.
FIG. 1 is a schematic cross-sectional view of a conventional headphone, which primarily includes a shell cover 1 and a shell body 2. A chamber space 3 defined between the shell cover 1 and the shell body 2 receives a circuit board 4 and a speaker 5 therein. The shape and/or structure of the circuit board 4 are constructed to be tightly mated with an inner ring-shaped contour of the shell cover 1. The speaker 5 is electrically connected to the circuit board 4 and outputs digital audio signals from a media player (not shown). The outer face of the shell body 2 is covered by an ear cover 6. The ear cover 6 provides user a great contact feeling while the headphone is tightly contacted with user's ears. In addition, the ear cover 6 also makes the headphone able to closely contact with user's auricles so as to prevent the ambient noise from leaking into the user's ears.
The aforementioned headphone has a passive de-noising ability provided by the ear covers 6; however, the speaker 5 is divided into a front chamber and a rear chamber. The front chamber and the ear cover 6 are connected to the users' auditory meatus, and the rear chamber is connected to the back of the speaker 5. In addition, the inner structure configuration of the aforementioned headphone should be designed to obtain improved sound quality at low frequency bands. However, to achieve better sound quality at low frequency bands, the headphone of special structure needs to overcome some space structure limits.